1. Field of the Invention
The present invention relates to hydraulic systems, and in particular to such systems that control hydraulic actuators on a work vehicle to maintain a predefined angular position of a load carrier as the load is raised and lowered.
2. Description of the Related Art
Construction and agricultural machines employ hydraulic systems to operate different mechanical elements. For example, a telehandler is a common material handling machine that has a pair of forks or a platform attached to the end of a telescopic boom pivotally attached to a tractor. Separate hydraulic actuators are employed to change the pivot angle, i.e. raise and lower the boom, and change the length of the boom, with each of those operations being referred to as a “function” of the machine. Another hydraulic actuator tilts the forks or platform with respect to the boom. The term “hydraulic actuator”, as used herein, generically refers any device, such as a cylinder or a motor, that converts hydraulic fluid flow into mechanical motion.
Historically, the machine operator controlled each function independently by manipulating different control levers. Each control lever was mechanically linked to operate a valve that governed the flow of pressurized fluid from a pump to the associated hydraulic cylinder and the return flow of fluid to a tank that supplied the pump.
There is a present trend away from mechanically operated hydraulic valves toward electrical controls using solenoid operated valves and control levels of electrical joysticks. Electrical control systems enable the valves to be located remotely from the operator cab, such as on the cylinder being controlled for example. This simplifies the hydraulic plumbing on the machine, as only a single pair of supply and tank return lines need to be run to service all the hydraulic actuators on a boom. Electrical operation of the valves also enables computerized control of the machine functions to provide additional capabilities that are unachievable with purely mechanical controls.
The operator may have to manipulate several control levers simultaneously in order to perform a given machine operation. In the case of a telehandler for example, as the boom is being raised or lowered, the position of the load carrier with respect to the boom must be varied to maintain the load carrier level and prevent a load from sliding off. Therefore, as the operator controls the hydraulic actuator that varies the boom pivot angle, the hydraulic actuator for the load carrier tilt also must be operated in a precisely controlled manner. Such simultaneous, coordinated maneuvers can be difficult to perform, especially by inexperienced operators.
Because it is desirable to simplify the operator control, several mechanical or hydraulic mechanisms have been developed to automatically maintain the load carrier level while the boom is being raised or lowered. For example, U.S. Pat. No. 4,767,256 discloses a second cylinder connected between the boom and the base on which the boom is mounted. As the boom moves up and down, fluid is exchanged between the second cylinder and the load carrier cylinder which causes the load carrier to pivot with respect to the boom in a manner that maintains a constant orientation of the load carrier with respect to the base.
Nevertheless, it is desirable to provide an electronic system that automatically maintains the load carrier level while the boom is being raised or lowered, without the need for an additional hydraulic cylinder.